Venetian type blinds have a series of slats hung on ladders which extend from a headrail to a bottomrail. Depending upon the width of the blind, two or more lift cords are provided. Usually, each lift cord is attached at one end to the bottomrails. The lift cords pass through elongated holes in the slats up to and through the headrail. A cord lock is usually provided in the headrail through which the lift cords pass. The cord lock allows the user to maintain the blind in any desired position from fully raised to fully lowered.
The slats rest on rungs between rails of the ladders. The blind is in an open position when the rungs are horizontal. To close most venetian blinds one lifts one rail while allowing the other rail to either remain in place or be lowered. This raises one end of each rung tilting the slats. Thus, when the blind is closed there is typically tension on one rail while the other rail of the ladder is not in tension.
The rails originally used for venetian blinds consisted of a fabric tape typically from one to two inches in width. The rungs were also made of fabric strips. Conventionally, the lift cords pass through holes in the slats which were aligned between the rails of the ladder. When the blind was closed, the fabric rails covered the holes. Consequently, there was no concern about light passing through the holes when the blind was in a closed position.
Walker in U.S. Pat. No. 2,572,224 discloses slats for a venetian type blind having a tape type ladder system and two lift cords. One lift cord passes through notches on the inside edges at one end of the slats. The second lift cord passes through notches on the outside edge of the other end of the slats. Each lift cord is positioned behind one of the tape-type ladders. Walker does not disclose any lift mechanism inferring that a standard lift mechanism could be used. In a blind made with Walker's slats the lift cords play no role in maintaining the slats in alignment when the blinds are closed. Rather, the tape-type rails of the ladders maintain the slats in alignment.
Very few tape type ladders are used in blinds today. Rather, the art has adopted ladders formed of cord rails having cord-type rungs between them. Typically, the rungs are multiple strands of cord. However, a single strand of cord has also been used. Frequently, a braided cord is used for the ladder rails. In nearly all venetian type blinds, the conventional lift cord arrangement in which the lift cord passes through elongated holes in the center of the slats is used. The lift cords are aligned to be between the rails of the ladders which are provided near opposite ends of the blind. When this type of blind is closed light can pass through the holes in the blind slats. This is readily apparent to anyone looking at the closed blind.
In pleated shades efforts have been made to prevent light from passing through the cord holes by making the holes to be a diameter very close to the diameter of the cord. This is not possible in a venetian type blind where the holes must be elongated to allow the slats to tilt properly.
In the venetian blinds which utilize a cord-type ladder the blinds are opened and closed by lifting or lowering one of the ladder rails relative to the other. In most blinds the lateral position of the ladder rails relative to one another remains unchanged at the headrail. Thus, there will be slack at the top of one of the ladder rails and the center of gravity shifts forward or back when the blind is tilted.
In my U.S. Pat. No. 5,573,051, I provide a venetian type blind having slats that are notched to form slots on their inside edge and outside edge to accommodate the lift cords. The lift cords pass from the bottomrail to the headrail each passing through a slot on the inside edge or outside edge of each slat. At least two cord-type ladders are provided to hold the slats and are aligned such that the rails of the ladders are adjacent the lift cords and outside the slots cut in the slats. At least two front slots are provided on the front edge of each slat and a corresponding set of rear slots are cut into the back edge of each slat opposite the front slots. The slots are sized so that the lift cords and the ends of the rungs can loosely fit therein. Because lift cords travel within the slots provided in the slats no cord holes are needed in the slats. Since there are no holes in the slats and the slots overlap the slats, light is effectively prevented from passing through the blind when the slats are rotated to a closed position. However, the slats in this blind can flip over which is not desired, particularly if curved slats are used.
Recently, Hunter Douglas introduced a venetian blind wherein the lift cord holes are punched close to the back edge of each slat. A single lift cord passes through each hole. Since the cord holes are off-center and the slats are curved transversely, the slats will close tightly only when tilted in a direction where the holes are on the upper edge of the titled slat. If tilted so that the cord holes are on the lower edge of the slat the edges of the curved slats hit the cord allowing a noticeable amount of light to pass through the blind when the slats are closed. Unlike the usual case where the holes are centered, in a blind having cord holes in the back edge of the slats the slats will not rotate in one direction in the same manner as they rotate in the other direction. Another problem occurs when a blind having lift cords passing through the back of the slats is raised. Because the support is off center and towards the back of the blind, when the blind is drawn up it has a tendency to arch backwards with the stacked slats forming a backward J shape. Consequently, the manufacturer of this blind provides installation instruction which tell the installer to position the blinds a greater certain distance from the window to account for this arching effect. Furthermore, the slats when lifted literally hang from the lift cords. Because the bottomrail does not support the tilted stack very well, more lift cords and heavier lift cords are required. Indeed, a lift cord is needed at every ladder. Since the slats hang from the lift cords, repeated titling of the slats wears the lift cords.
Accordingly, there is a need for venetian blind which can have slats with cord holes at only one edge of the slats in order to prevent light from entering through the holes when the slats are rotated closed and prevent the slats from flipping. Such a blind should overcome the disadvantages in the prior art by closing tightly when the slats are rotated in either direction and also should maintain vertical alignment of the slats when the blind is fully raised. In this blind tilted slats should not hang on the lift cords, but be supported by the ladders and bottomrail. Then smaller diameter lift cords can be used which allows for narrower holes and better closure when the blind is tilted with the holes down.